As a result of advances in satellite technology and the growing number of applications which utilize satellites, satellite constellations are becoming increasingly important. For example, satellite constellations may be used to provide global coverage or near global coverage for communications, environmental monitoring, weather forecasting, or surveillance purposes. In addition, advances in satellite technology, launch vehicles and facilities, sensor technologies, on-board microprocessors, and satellite communications have made satellite constellations more commercially feasible.
The increasing demand for satellite constellations, in turn, has resulted in greater emphasis on direct communications, or cross linking, between satellites within a constellation. For example, cross linking may be used to efficiently coordinate satellites within a constellation. Two or more satellites might communicate with each other in order to coordinate their tracking of a common location, thus resulting in stereo tracking of that location.
Cross linking may also be used to maintain a communications network among the satellites of the constellation. For example, if the purpose of the constellation is to provide a global communications network similar in architecture to the land-based cellular telephone network, then cross linking may be used to pass messages across the satellite constellation. As another example, satellites in a constellation may acquire raw data which is to be processed via on-board processors before down-linking to a ground station, thus reducing the amount of data to be down-linked. The desired processing, however, may require data from several satellites. Cross linking may be used to centralize the raw data in a single satellite, thus facilitating on-board processing. If the single satellite also handles communications between the constellation and the ground station, then the total number of ground stations and/or the sophistication of the ground station may be reduced.
The demand for satellite constellations has also resulted in increased interest in low earth orbits. For example, the altitude of geosynchronous orbit may be excessive for communications or sensor range to the ground. However, as satellite altitudes are reduced, it becomes more difficult to communicate between satellites because a large part of the view of each satellite is blocked by the earth. Inter-satellite cross linking also becomes more complex while the need becomes greater since ground link relays become more impractical.
Conventional satellite constellations utilize plane-based cross link architectures. In this architecture, the constellation contains a number of orbits, each of which lies in a distinct plane. Each orbit contains enough satellites to allow communication among all the satellites in the orbit without earth blockage. Communication between satellites in two different orbits is centered about the intersection of the two orbits. As satellites from the two orbits approach the intersection point, they become close enough to allow cross linking between the two satellites and hence the two orbits. The cross link allows communications between the two satellites for as long as the cross link is operational. Since each of the two satellites can communicate with any of the other satellites in its orbit, the cross link essentially allows communications between the two orbits. However, as the two cross linked satellites travel along their orbits, they will begin to travel away from the intersection point and each other and, at some point, the cross link between the two satellites can no longer be maintained. However, the cross link will be replaced by a new cross link between two other satellites approaching the intersection point. In this way, any two orbits and hence the entire satellite constellation may be permanently networked together. The making and/or breaking of cross links is referred to as switching.
The plane-based cross link architecture, however, has several disadvantages. First, switching, which is not an insignificant task and may require additional equipment on the satellites, is required to maintain the communications network. In addition, since the network is always being reconfigured, routing of messages is dynamic and must be continuously updated. These problems can affect the reliability of the network and multiply as the number of orbits increases. Another disadvantage of plane-based cross link architectures is that at low altitudes, the communications range of any one satellite is significantly limited by the earth's curvature. As a result, in order to maintain communications among all the satellites in a specific orbit, as is required by the plane-based cross link architecture, each orbit must contain a large number of satellites. The resulting plane-based constellation requires many satellites.
Thus, there is a need for satellite constellations which support cross linking architectures based on reduced or no switching. There is also a need for satellite constellations which utilize fewer numbers of satellites at low altitudes.